North Korea may still declare all of its nuclear weapons and disarm them by the end of this year despite no agreement on a deadline being reached at six-nation talks, says the US envoy.

North Korea may still declare all of its nuclear weapons and disarm them by the end of this year despite no agreement on a deadline being reached at six-nation talks, the US envoy said on Friday.

"My opinion remains the same. All of this is do-able by the end of the year," Christopher Hill told reporters when asked about the failure to set the ambitious deadline during the three days of discussions in Beijing.

The United States went into the talks saying it wanted North Korea to make a complete declaration of all its nuclear weapons programmes and disable them by the end of the year.

The "declare and disable" phase was the second plank of a six-nation disarmament accord brokered in February under which North Korea would eventually completely abandon its nuclear weapons programmes.

In return, it would get one million tons of fuel oil or equivalent energy aid, as well as wide ranging diplomatic concessions and security guarantees.

The talks began amid optimism after North Korea fulfilled its first commitments under the February accord by closing all five facilities at its main nuclear reactor complex.

South Korean envoy Chun Yung-Woo then told reporters after Wednesday's talks that North Korea was willing to disarm and disable by the end of the year.

However all parties were unable to agree on the firm timetable, with host China having the final say in deciding not to include the deadline in a statement that is due to be released later Friday, according to Hill.

"It was the decision of the Chinese chair not to include that opinion," he said of the deadline.

Nevertheless, Hill said this week's talks had been extremely positive.

"This session absolutely built momentum. Look how far we have come in the last seven days," he said, referring also to the shutdown of North Korea's Yongbyon reactor complex.

South Korea's Chun agreed that the latest round of the long-running talks had been a success.

"The biggest achievement this time is that North Korea has clearly expressed its intention not to delay the implementation of the February 13 agreement," Chun said.

"North Korea has said it intends to fulfill its obligations, if conditions are met, as quickly as possible under the agreement."

Hill and Chun said a final deadline could still be set after all parties broke into working groups to hammer out the details of what needs to be declared and how to close them.

The working groups are expected to meet over the next few weeks, with another round of six-party talks tentatively scheduled for late August, according to Hill.

The six-nation talks began in 2003 with the aim of convincing North Korea to abandon its nuclear ambitions. They group China, the two Koreas, the United States, Japan and Russia.